How do I deauthorize a computer from iTunes that I no longer have?

If you no longer have it then you have to deauthorize all and re-authorize the ones you do have.

iTunes Store: About authorization and deauthorization - http://support.apple.com/kb/HT1420 - and another helpful post: https://discussions.apple.com/thread/3798085?answerId=17828050022#17828050022

About deauthorizing specific computers - https://discussions.apple.com/thread/3809988 - Yes, if you have physical access to the computer.